Where as the following combinations get connected:
connect OPS$DT-SAP-CRM\SAPSERVICE<SID> /Password
OPS$DT-SAP-CRM\<SID>ADM /Password
SAPSR3/Password
The above 3 pairs are working well, only connect /@<SID> is giving 1027 error.
I have followed all the steps in Note 400241
As per the note we have added:
REMOTE_OS_AUTHENT = TRUE
os_authent_prefix = ops$

We also deleted and recreated the OPS$ users. Still we are facing teh same issue. The connect /@<SID> is failing and R3trans is also failing with this error.

Hi
The OPS$ users are not created properly.
SAP doesn't care whether you are able to connect to the database with user and password from the SQL prompt.
Please follow this note and fix your problem :
Note 400241 - Problems with ops$ or sapr3 connect to Oracle